This invention relates to a sag compensation circuit for a video signal for compensating a sag of the video signal caused by loss of a low frequency component of the video signal.
If an input video signal includes a sag component, a luminance gradient, which is not included in the original picture, appears in a reproduced video signal on a display or the like and results in a degradation of picture quality. In order to improve the picture quality, the sag must be compensated.
Referring to FIGS. 1 and 2, two examples of a conventional sag compensation circuit will now be described.
FIG. 1 shows a block diagram of a first conventional sag compensation circuit. The first conventional circuit consists of:
a clamp circuit 2 for clamping a tip, or a peak of a synchronization signal of an input video signal 1 to a reference voltage in accordance with a generated synchronization signal and for outputting an output video signal 8; and PA1 a synchronization signal generator 3 for generating the generated synchronization signal based on the input video signal. PA1 a capacitor 21 having one electrode connected to the input video signal and another electrode connected to one end of a switch 23; PA1 a Clamp Pulse Generator (CPG) 22 for generating a clamp pulse signal in accordance with the generated synchronization signal; PA1 a Reference Voltage Source (RVS) 24 for supplying the reference voltage; and PA1 the switch 23, the other end of which is connected to the reference voltage source, the switch 23 opens or closes in accordance with the clamp pulse signal. PA1 an adder 95 for adding an amplified signal of a DC amplifier 98 to an input video signal 1; PA1 a DC inverting amplifier 96 for inverting and amplifying an output of the adder 95 and outputting an output video signal 8; PA1 a sync-tip detector 97 for detecting a voltage of a tip of a synchronization signal of the output video signal 8 and for holding the voltage until the arrival of the next tip of the synchronization signal of the input video signal; and PA1 the DC amplifier 98 for amplifying the held voltage of the sync-tip detector 97 and for outputting the amplified signal. PA1 a synchronization signal generator for generating a synchronization signal in synchronism with a video signal synchronization signal; PA1 a first clamp circuit for clamping a predetermined portion of the video signal synchronization signal to a fixed level in accordance with the generated synchronization signal and for outputting a clamped video signal; PA1 a level detector for detecting a level of the predetermined portion of the clamped video signal synchronization signal in accordance with the generated synchronization signal and for outputting the level as a detected level; PA1 a converter for converting the detected level to a reference level signal so that the larger the detected level becomes the smaller the reference signal becomes; and PA1 a second clamp circuit for clamping the predetermined portion of the clamped video signal to the reference level signal in accordance with the generated synchronization signal and for outputting an output video signal .
The clamp circuit 2 includes:
The synchronization signal generator 3 generates the generated synchronization signal which synchronizes with the synchronization signal of the input video signal. In response to the generated synchronization signal, the clamp pulse generator 22 generates the clamp pulse signal which causes the switch 23 to close at the tip of the synchronization signal of the input video signal. By the operations of the switch 23 and the capacitor 21, a predetermined portion of an output video signal 8 (for example, a tip of a synchronization signal of the output video signal) is clamped, or level-shifted to near the reference voltage of the reference voltage generator 24.
However, the first conventional sag compensation circuit has a problem that a small residual sag component remains in the output video signal. This is because a resistance component of the switch 23 prevents a tip level of a synchronization signal of the output video signal from being set to the reference voltage accurately.
FIG. 2 shows a block diagram of a second conventional sag compensation circuit, which is described in Japanese Laid-Open Patent Publication No. 62-272766 (1987).
As shown in FIG. 2, the second conventional circuit includes:
The input video signal 1 is added to an output of the DC amplifier 98 in the adder 95, an output which is inverted and DC-amplified in the DC inverting amplifier 96 in order to output the output video signal 8. The output video signal is also supplied to the sync-tip detector 97. The sync tip detector detects a DC level of a tip of a synchronization signal of the output video signal and holds the DC level until the arrival of the next tip. The DC amplifier amplifies the DC level and supplies the amplified signal to the adder 95.
When the tip of the input video signal 1 increases, the amplified signal of DC amplifier 98 decreases and vice versa, because the DC inverting amplifier 96 and the DC amplifier 98 have a different polarity of amplification relative to each other. Thus, the sag in the input video signal can be compensated.
The second conventional circuit overcomes the problem of the first conventional circuit. However, the second conventional circuit has the following new problem. In the second conventional circuit, when an impulse noise or the like is included in the detected level, the impulse noise appears in the adder 95 output and circulates in the closed loop of FIG. 2. This circulation degrades the picture quality. Therefore, the second conventional circuit does not accurately compensate for the sag when the input video signal is noisy.